Drive units and drive assembly for a reciprocating slat conveyors

ABSTRACT

Three tandem drive units are provided. Outward variable volume working chambers at one end of the drive units are connected via passageways in the piston rods with inward variable volume working chambers at the opposite end of the drive units. In similar fashion, inward variable volume working chambers at the first ends of the drive units are connected to outward variable volume working chambers at the second ends of the drive units, also via passageways in the piston rods. At each end of the assembly, the piston rods pass through piston rod receiving openings in a cylinder head forming member. Each cylinder head forming member includes a manifold passageway and spool valves for controlling a sequencing movement of the drive units. The piston rods provide the spools for the spool valves.

TECHNICAL FIELD

This invention relates to improvements in reciprocating slat conveyorsand, in particular, to the provision of an improved drive assembly thateliminates most of the external hoses that are essential in the priorart drive assemblies of its type. It is also relates to the provision ofimproved linear hydraulic drive units.

BACKGROUND ART

Reciprocating slat conveyors powered by linear hydraulic motors becamemore than a curiosity in about 1974 when it was discovered thatsequencing could best be controlled by controlling the release of oilfrom the motors rather than the delivery of oil to the motors. The nextsignificant discovery was the discovery that drive units could be usedas beams and be subjected to an applied side load contrary to theprevious wisdom of the art that linear hydraulic drive units should notbe side loaded. This discovery led to the development of a reduced sizedrive assembly, an early example of which is disclosed in U.S. Pat. No.Re. 35,022, granted Aug. 22, 1995, to Raymond K. Foster, and entitled,"Reduced Size Drive/Frame Assembly For A Reciprocating Floor Conveyor."

An effort has been made to use terms herein that will name the driveassembly components without imposing detail or limitations that are notnecessary to the invention. A "linear hydraulic motor" is but one ofmany names given to a hydraulic device that is composed of piston andcylinder components, one of which is fixed and the other of which isallowed to move back and forth along a linear path. Movement is causedby introducing and removing hydraulic fluid pressure to and fromvariable volume working chambers formed by and between the fixed andmovable components. "Linear hydraulic motors" are also termed "linearactuators", "linear hydraulic drive units", and simply "hydrauliccylinders". A problem with calling them "hydraulic cylinders" when it isnecessary to describe them in some detail is that one of the componentsof the device is also referred to as a "cylinder" or "cylindercomponent." Herein, the "linear hydraulic motors" that will be describedare broadly referred to as "linear hydraulic drive units." The pistoncomponents of these drive units are composed of piston rods and pistonheads. Each cylinder component includes a tubular or cylindricalsidewall, a cylinder head at one end through which the piston rodextends, and an end wall at the opposite end. Herein, the term "cylindertube" is used to name the cylinder sidewall.

U.S. Pat. No. 4,821,898, granted Apr. 18, 1989 to Raymond K. Foster, andentitled "Drive/Frame Assembly For A Reciprocating Floor" discloses adrive assembly that is similar to the drive assembly disclosed in theaforementioned U.S. Pat. No. Re. 35,022 except that its drive units area tandem type having fixed opposite end portions that are pistoncomponents and a movable central portion that is a cylinder component.This particular type of tandem drive unit eliminates external hoses byincorporating some of the fluid passageways in the piston rods.

U.S. Pat. No. 4,691,819, granted Sep. 8, 1987, to Olof A. Hallstrom, andentitled, "Reciprocating Conveyor", discloses tandem drive units havingfixed cylinder components at their ends and a movable central pistoncomponent. A problem with this drive unit is that it requires the use ofa considerable amount of external hoses. In a commercial embodiment,manufactured by Hallco Mfg. Co., Inc. of Tillamook, Oreg., named theHallco model 4000 conveyor, the cylinder components are of a tie boltconstruction and include a manifold at each end. The inward manifoldsinclude some fluid passageways. However, hoses are used to connect thetwo manifolds at one end of the drive assembly to the two manifolds atthe opposite end of the assembly. This construction of the Hallco model4000 conveyor is disclosed in a Hallco owner's manual that is publishedby Hallco Mfg. Co., Inc.

Other prior art drive assemblies utilizing tandem linear hydraulicmotors are disclosed in U.S. Pat. No. 5,638,943, granted Jun. 17, 1997to Raymond K. Foster, and entitled "Drive Assembly For ReciprocatingSlat Conveyors." FIG. 4 of this patent discloses a drive assembly thatis marketed in Europe by Cargo Handling Systems, B. V., located inCoevorden in the Netherlands. This system utilizes passageways in thepiston rods in lieu of some of the hoses but continues to use hoses orconduits for connecting the outward ends of the hydraulic drive unitswith each other and with a main operating valve, or switching valve, andwith a directional control valve. The directional control valve,together with other components of the system, adapts the conveyor forboth two way loading and unloading.

All of the prior art drive assemblies that utilize tandem drive unitsalso utilize poppet type sequencing valves that are either built intothe cylinder components or provided as an auxiliary structure. Theaforementioned U.S. Pat. No. 4,821,863 shows sequencing valves that areauxiliary structure. The Hallco Model 4000 conveyors have check valvesbuilt into its inner manifolds. These check valves are essentially likethe check valves disclosed in U.S. Pat. No. Re. 35,022. Cargo HandlingSystems, B. V. builds poppet type sequencing valves into the outwardends of the cylinder components.

Richard T. Gist of Tulare, Calif., has developed a drive assembly forreciprocating slat conveyors which uses spool valves for sequencing inplace of poppet type check valves. Portions of the piston rods providethe valve spools. However, this drive assembly continues to require theuse of external hoses or pipes.

An object of the present invention is to provide an improved driveassembly in which poppet sequencing valves and external hoses areessentially eliminated. The valve that switches the system betweenloading and unloading modes is incorporated into a housing that isintegrated with a cylinder head forming member. The piston rods andpiston rod openings in cylinder head forming members together providethe sequencing valves.

Another object of the invention is to provide a conveyor drive that issimple in construction, is durable in use, is efficient in operation,and can be economically manufactured.

BRIEF SUMMARY

The drive units herein described are elongated in a direction that isparallel to the conveyor slats that are reciprocated by the drive units.In describing the drive units, and the parts thereof, the term "outward"means directed outward in the longitudinal direction. The term "inward"means extending longitudinally in the direction opposite the outwarddirection. The terms "first" and "second" are used to identify which oftwo elements of a kind are being discussed. In describing a linearhydraulic drive unit, for example, either end, or a component at eitherend (e.g. an outward variable volume working chamber) will be the"first" if it is described first and the other end or like componentwill be the "second." Most of the time, it makes no difference which endor component is considered to be the "first" and which is considered tobe the "second."

A linear hydraulic drive unit of the invention has a first outwardvariable volume working chamber and a first inward variable volumeworking chamber at a first end of the drive unit. It also has a secondoutward variable volume working chamber and a second inward variablevolume working chamber at the second end of the unit. A first internalpassageway extends from the first outward variable volume workingchamber through a first piston head, then through a piston rod, and thenthrough a second piston head to the second inward variable volumeworking chamber. A second internal passageway extends from the firstinward variable volume working chamber, then through the first pistonhead, then through the piston rod, and then through the second pistonhead to a second outward variable volume working chamber. The piston rodextends through piston rod receiving openings in a pair oflongitudinally spaced apart cylinder heads. Each cylinder head includesan inlet/outlet port, a fluid passageway extending from the inlet/outletport into the cylinder head, and a chamber port connecting thepassageway with the inward variable volume working chamber at its end ofthe drive unit.

According to an aspect of the invention, the inward variable volumeworking chamber at one end of the drive unit is connected by an internalpassageway with the outward variable volume working chamber at theopposite end of the drive unit. The outward variable volume workingchamber at the first end of the drive unit is connected by a differentinternal passageway to an inward variable volume working chamber at thesecond end of the drive unit. All fluid pressure flow into and out fromeach connected together pair of inward and outward variable volumeworking chambers is via a chamber port that connects the inward variablevolume working chamber of the pair with the passageway in the cylinderhead, and the inlet/outlet port for such passageway.

According to another aspect of the invention, three tandem linearhydraulic drive units of the type described positioned side-by-side andtheir cylinder heads are made into a single member that in preferredform is a block of metal that is basically aluminum. The cylinder headforming member or block includes a piston rod receiving opening for eachpiston rod. A manifold passageway in the piston head forming memberextends from the inlet/outlet port, first to one side of the closestpiston rod receiving opening, and then from an opposite side of the samepiston rod receiving opening to the near side of the next piston rodreceiving opening, for the middle piston rod, and then from an oppositeside region of the piston rod receiving opening for the middle pistonrod, onto a chamber port for the third drive unit. According to thisaspect of the invention, the piston rod receiving opening and the pistonrod together form a spool valve that either permits or blocks fluidpressure flow in the manifold passageway across the piston rod receivingopening in the cylinder head forming member.

According to another aspect of the invention, a cylinder head member isprovided which includes internally threaded sockets, one for eachcylinder tube, entering into the cylinder head on the side thereofopposite the cylinder tubes. A metal sleeve is provided for each pistonrod. Each metal sleeve has an externally threaded nipple that threadsinto a threaded socket. The sleeve includes an internal girth groove inwhich a seal ring is received, for sealing around the piston rod,between it and the interior of the sleeve. In preferred form, eachcylinder head is made from a metal that is primarily aluminum. Thesleeve is made from a structurally stronger metal, such as a metal thatis basically steel. In preferred form, the sleeve includes a secondinternal girth groove in which a wear ring is received. The wear ringsurrounds the piston rod and contacts the outer surface of the pistonrod when the piston rod reciprocates back and forth during use. Inpreferred form, each sleeve also includes another internal groove at itsinward end in which a dirt seal is received. Each sleeve is secured tothe cylinder head forming member and its wear ring makes most of thewear contact with the piston rod. In preferred form, the portion of thecylinder head forming member that is between the sleeve and the inwardvariable volume working chamber is what functions as a spool valvetogether with the piston rod, the piston rod receiving opening in thecylinder head, and the passageway in the cylinder head that intersectsthe piston rod receiving opening.

Another aspect of the invention is to provide a drive unit having atleast one cylinder head forming member that includes an internallythreaded socket for receiving an externally threaded inward end portionof a cylinder tube. This connection of the inward end of the cylindertube with the cylinder head provides an inward end closure for the spacethat is defined in and by the cylinder tube. The outer end of thecylinder tube may be threaded (e.g. internally) to receive a threaded(e.g. externally) nipple on an end cap. Or, the outward end of the spaceformed in and by the cylinder tube may be closed in some other manner.The threaded connection is preferred, particularly when the inward endof the cylinder tube is thread connected to the cylinder head, so thattie bolts or the like are unnecessary.

These and other advantages, objects, and features will become apparentfrom the following best mode description, the accompanying drawings, andthe claims, which are all incorporated herein as part of the disclosureof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference numerals and letters refer to like partsthroughout the several views and:

FIG. 1 is a pictorial view of a drive assembly for a reciprocating slatconveyor, looking from below towards one end, the bottom and one side ofthe assembly;

FIG. 2 is a top plan view of the drive assembly shown by FIG. 1;

FIG. 3 is a sectional view taken substantially along line 3--3 of FIG.2;

FIG. 4 is a bottom plan view of the drive assembly shown by FIGS. 1-3,with opposite end portions of the transverse drive beams omitted;

FIG. 5 is an exploded pictorial view of the components of the driveassembly shown by FIGS. 1-4, minus the transverse drive beam, the centerportions of the piston rods and the connectors for connecting thetransverse drive beams to the piston rods;

FIG. 6 is an enlarged scale exploded pictorial view of the left endportion of FIG. 5;

FIG. 7 is an enlarged scale exploded pictorial view of the left endcylinder head member and the valve components below it, shown in FIG. 5;

FIG. 8 is an enlarged scale pictorial view of the portion of FIG. 5between the right end cylinder head member and the piston rods;

FIG. 9 is an enlarged scale exploded pictorial view of the portion ofFIG. 5 that includes the piston rods and the components between thepiston rods and the left end cylinder head member;

FIG. 10 is an enlarged scale pictorial view of the right end cylinderhead member and the valve components below it, in FIG. 5;

FIG. 11 is an enlarged scale pictorial view of the right end cylinderhead member and the remaining components of the assembly that arepositioned to the right of the right end cylinder head member;

FIG. 12 is an enlarged scale longitudinal sectional view of one of theleft end cylinder assemblies in FIG. 5;

FIG. 13 is a cross sectional view taken substantially along line 13--13of FIG. 12;

FIG. 14 is a cross sectional view taken substantially along line 14--14of FIG. 12;

FIG. 15 is a right side up cross sectional view taken substantiallyalong line 15--15 of FIG. 4;

FIG. 16 is a right side up cross sectional view taken substantiallyalong line 16--16 of FIG. 4;

FIG. 17 is a sectional view taken substantially along line 17--17 ofFIG. 3, showing full diameter portions of the piston rods within thepiston rod openings in the cylinder head;

FIG. 17A is a view like FIG. 17, but showing reduced diameter portionsof the middle and right side piston rod in alignment with the manifoldpassageway in the cylinder head forming member;

FIG. 18 is a longitudinal sectional view taken through the end portionof a drive unit at the end of the assembly opposite from the end thatincludes the end portion shown by FIG. 12;

FIG. 19 is a cross sectional view taken substantially along line 19--19of FIG. 18;

FIG. 20 is a cross sectional view taken substantially along line 20--20of FIG. 12;

FIGS. 21-30 are sequential views in schematic form, with FIG. 21 showingall three of the piston rods moving together towards the rear of theconveyor, for advancing a load that is on the conveyor;

FIG. 22 shows all three piston components at a fully advanced position;

FIG. 23 shows the first piston component retracting while the second andthird piston components are at rest at the advanced position;

FIG. 24 shows the first piston component at rest at a retractedposition, shows the second piston component retracting and shows thethird piston component at rest at an extended position;

FIG. 25 shows all three piston components in retracted positions;

FIG. 26 shows all three piston components moving together towards thefront of the conveyor, for loading a load;

FIG. 27 shows all three piston components at rest at a fully advancedposition;

FIG. 28 shows the third piston component retracting and the first andsecond piston components at rest in the advanced position;

FIG. 29 shows the third piston rod component at rest in a retractedposition, the second piston component retracting and the third pistoncomponent at rest in a fully advanced position;

FIG. 30 shows the first piston component retracting and the second andthird piston components at rest in a fully retracted position;

FIG. 31 is a schematic diagram of the entire system, with the directioncontrol valves positioned for causing an unloading movement of theconveyor slats; and

FIG. 32 is an enlarged scale view of the valve assembly shown at theupper left portion of FIG. 31.

BEST MODE FOR CARRYING OUT THE INVENTION

The principles that constitute the present invention can best beunderstood by considering an embodiment of the invention and inparticular the best mode of the invention that is shown by the drawing.

Referring first to FIG. 1, a drive assembly 10 embodying the presentinvention is shown to comprise three hydraulic drive units 12, 14, 16.Each drive unit 12, 14, 16 has fixed opposite end portions and a movablecenter portion. The end portions are secured to frame structure in somesuitable manner and the movable center portions are connected totransverse drive beams 18, 20, 22. Preferably, the transverse drivebeams 18, 20, 22 are connected to the drive units by use of theconnection structure C that is disclosed in my co-pending applicationSer. No. 09/322,271, filed May 28, 1999, and entitled "Drive Beam toDrive Unit Connections."

In the preferred embodiment, transverse drive beam 18 is connected topiston rod 24 for drive unit 12. Transverse drive beam 20 is connectedto piston rod 26 for drive unit 14. Transverse drive beam 22 isconnected to piston rod 28 for drive unit 16. Referring to FIG. 2, a topplan view, transverse drive beam 18 is provided with a plurality ofconnectors 30, one for each conveyor slat that is connected to the drivebeam 18. In like fashion, transverse drive beam 20 is provided with aplurality of conveyor slat connectors 32 and transverse drive beam 22 isprovided with a plurality of conveyor slat connectors 34.

As is well known to persons skilled in the art, conveyor slats areconnected to the connectors 30, 32, 34, such as in the manner shown byU.S. Pat. No. 4,492,303, granted Jan. 8, 1985 to Raymond Keith Foster,and entitled "Drive/Guide System For A Reciprocating Floor Conveyor". Asalso shown in that patent, the conveyor slats are supported on and byself lubricated plastic bearings which are themselves supported bylongitudinal support beams, shown in that patent in the form of squaretubes.

The drive units 12, 14, 16 are tandem drive units, composed of pistoncomponents and cylinder components. The piston components include thepiston rods 24, 26, 28, and piston heads to be described later, at bothends of the piston rods 24, 26, 28. The cylinder components arepreferably provided by a pair of cylinder tube assemblies 36, 38, one ateach end of the drive assembly 10. In the illustrated embodiment, eachcylinder tube assembly includes a cylinder head forming member 40, 42and a plurality of cylinder tubes 44, 46, 48, and 50, 52, 54, one foreach drive unit.

The cylinder tube assemblies 36, 32 are secured in any suitable mannerto a frame structure that might be a portion of the drive assembly 10,and used for mounting the drive assembly 10 in an installation frame.Or, the cylinder tube assemblies 36, 38 may be directly connected to aninstallation frame. By way of typical and therefore non limitiveexample, several forms of a mounting frame for the drive assembly isshown by the aforementioned U.S. Pat. No. 5,638,943 (FIGS. 1, 2 and5-12).

FIG. 5 herein is an exploded pictorial view of most of the componentsthat in combination make up the drive units 12, 14, 16. The primarypurpose of FIG. 5 is to orient the enlarged scale fragmentary portionsof FIG. 5 that are shown by FIGS. 6-11. For that purpose, very fewreference numerals are used in FIG. 5. Primarily, the componentsappearing at the ends of FIGS. 6-11 are provided with reference numeralsin FIG. 5. Cylinder head members 40, 42 appear in most of the views.This is done to further provide a way of orienting FIGS. 6-11 to FIG. 5and to each other.

The drawings all form a part of the description of the invention. FIGS.5-11 show parts that are readily identifiable and for that reason do notneed to be described. Also, the figures of the drawing constituteschematic diagrams for the principles that are exemplified by thestructure that is illustrated and described. Accordingly, when studyingthe drawings and reading this document it should be kept in mind thatthe components that are shown and described by way of example alsorepresent other components that exist that are equivalent for performingthe same function as the illustrated part. Referring to FIG. 6, thecylinder tubes 44, 46, 48 may have threads at each end. The threads atthe outward ends of the cylinder tubes 44, 46, 48 are designated 56.They are preferably internal threads adapted to receive threaded endwardend portions 58 of end cap members 60, 62, 64. End cap members 60, 62,64 provide outward end wall for the inner spaces in the cylinder tubes44, 46, 48. O-rings may be provided to seal between the threads 56, 58.

The inward ends of cylinder tubes 44, 46, 48 may be externally threadedat 66. This is so that they can screw into internally threaded sockets68 formed in cylinder head member 40. O-rings may be provided to sealbetween the mating threads 66, 68, as illustrated. Opposite theinternally threaded sockets 68, the cylinder head forming member 40 mayinclude smaller diameter, internally threaded sockets 70 (FIG. 12) sizedto receive externally threaded nipples 72 on sleeves 74. In a preferredembodiment, the cylinder head forming members 40, 42 are made from ametal that is primarily aluminum. The sleeves 74 are made from a metalthat is primarily steel or some other metal that is structurallystronger that aluminum. O-rings are provided to seal between the threads70 and the threads 72. As best shown by FIG. 12, each sleeve 74 includesan inner groove 76 for receiving a seal ring. Preferably it alsoincludes a second inner groove 78 for receiving a wear ring 80 (FIG. 7).Preferably also, it includes a third internal groove 82 for receiving adirt seal. As shown by FIG. 12, the sleeve 74 is threaded into thethreaded socket 70 with an O-ring seal in the space 84. The O-ring seal84 seals between the sleeve 74 and the cylinder head forming member 70.The seal within space 76 seals between sleeve 74 and the shaft. In FIG.12, the shaft that is shown is shaft 24.

Referring to FIGS. 8 and 11, cylinder head forming member 42 alsoincludes internally threaded sockets 88 for receiving externallythreaded nipples 90 on sleeves 92. Sleeves 92 may be identical tosleeves 74. Cylinder head members 40, 42 may be constructed from thesame material and sleeves 74, 92 may be constructed from the samematerial.

Referring to FIG. 11, the cylinder head member 42 includes sockets onthe side of it opposite sockets 88 that are provided for receiving theexternally threaded inward ends 90 of the cylinder tubes 50, 52, 54. Asbest shown by FIG. 18, O-rings are inserted over and about the threads90 and then the threads are inserted into the sockets 92 and thecylinder tubes 50, 52, 54 are rotated to thread them into the sockets90. The O-rings are compressed so that it seals against leakage at thethreads. The outward ends of the cylinder tubes 50, 52, 54 preferablyhave internal threads for receiving external threads on nipples 94 thatare apart of caps or plugs 96, 98, 100. O-rings are preferablypositioned on the nipples 94 and then the nipples 94 are inserted intothe threaded openings in the outward ends of the cylinder tubes 50, 52,54. Then the caps are rotated to thread them into the cylinder tubes 50,52, 54. When the end plugs 96, 98, 100 are tightened, the O-ring iscompressed and it serves to seal against leakage at the threads.

The piston component construction will now be described, primarily inconjunction with FIGS. 12-13 and 18-30, but with some reference to FIGS.5, 6, 8 and 9. Referring first to FIGS. 12-14 and 18-20, these figuresshow all of drive unit 12 except for the central portion of the pistonrod 24. FIGS. 1-4 show the full length of the drive unit assembly. Thecentral portion of the piston rod 24 that is omitted from FIGS. 12 and18 is like the portion of piston rod 24 that is shown in FIGS. 12 and18, where it passes through the cylinder head forming members 40, 42 andthe sleeves 74, 92.

All three drive units 12, 14, 16 are alike so it is only necessary toillustrate one of them. Drive unit 12 was selected for illustration. Itis drive unit 12 that is shown in FIGS. 12-14 and 18-20. Piston rod 24has a tubular body 102 that extends from a connection with piston head104 within cylinder tube 44 to a connection with a piston head 106within cylinder tube 50. Piston rod body 102 has a substantiallyconstant diameter center passageway 106. The end of piston rod body 102that is connected to piston head 104 is preferably externally threadedat 108. The piston head 104 includes a socket having complementaryinternal threads 110. An O-ring is positioned between the piston head104 and the end portion 108 and then the piston head 104 is threadedonto the end portion 108.

Piston head 104 has a somewhat annular outer portion 112 and a somewhatcylindrical inner portion 114. O-rings are positioned in the O-ringgrooves that are illustrated and then the annular portion 116 isconnected to the cylindrical portion 114. The annular portion 112 isformed to include a plurality of axial passageways 116. There are sixpassageways 116 in the illustrated embodiment (FIG. 14). Radialpassageways 118 in member 112 are in alignment with radial passageways120 in member 114. The inner ends of passageways 120 meet at a centerspace 122.

Piston rod 24 is provided with a center tube 124 that extends throughthe passageway 106 of member 102 from piston head 104 to piston head 106(FIGS. 12 and 18). Piston head 102 has an increased diameter socketportion adjacent the space 122. An end portion of tube 124 fits snuglywithin this socket. An annular passageway 126 is formed by and betweenthe tubular members 102, 124. Center portion 114 of piston head 104includes a plurality of axial ports 128. These ports 128 communicate theouter variable volume working chamber 130 with the annular passageway126 (FIG. 12). The radial ports 118 and the axial ports 116 communicatethe interior of tube 124 with the inward variable volume working chamber132. The piston head 104 divides the space within the cylinder tube 44into an outer variable volume working chamber 130 and an inner variablevolume working chamber 132.

As shown by FIG. 12, the piston head 104 preferably has three girthgrooves surrounding it. The two end grooves receive seals rings. Thecenter groove receives a wear ring.

At the opposite end of the piston rod 24, the threaded end portion 134of the piston rod member 102 engages internal threads 136 formed in anaxial socket formed in the piston head 106. The interior of this socket138 communicates with a plurality of axial ports 140. These axial ports140 communicate the socket 138 with the inward variable volume workingchamber 142. The outward end 144 of the piston head 106 is a radial wall144 that is formed to include a circular center opening 140. The rightend portion 148 of the tube 124 fits snugly within the opening 146. Aseal ring groove is shown formed in member 144, positioned to receive aseal ring that surrounds the tube 124. There is fluid communicationbetween the inside of tube 124 and the outward variable volume workingchamber 150.

Looking at FIGS. 12 and 18 together, fluid pressure in inward workingchamber 132 is communicated via the ports 116, and 120, and centercavity 122, in piston head 104, with the outward working chamber 150,via the interior of tube 124. In similar fashion, the outward workingchamber 130 is communicated with the inward working chamber 142 viaports 128 in piston head 104, via annular passageway 126 in the pistonrod 24, and via the center cavity 138 and the ports 140 in piston head106.

Referring to FIGS. 31 and 32, the switching valve SV is preferably likethe switching valve disclosed in U.S. Pat. No. 5,103,866, granted Apr.14, 1992, to Raymond Keith Foster, and entitled, "Poppet Valve An ValveAssemblies Utilizing Same". However, any suitable four-way switchingvalve may be employed. The control rod 152 may be moved in the mannerdisclosed in U.S. Pat. No. 5,103,866 (FIGS. 7-10). Contact withtransverse drive beam 18 moves the control rod 152 in one direction.Contact with transverse drive beam 22 moves it in the oppositedirection. The switching valve SV includes a pressure port 154 that isconnected to a pressure supply conduit 156 that extends from a source offluid pressure, e.g. a pump P. Valve SV also includes a return port 158that is connected to a return line 160 that returns hydraulic fluid to atank. Switching valve SV also includes two cylinder ports 162, 164. Aconduit 166 extends from port 164 to a port 168 in valve housing 170.Cylinder port 162 is connected to a conduit 172 that extends to a port174 in a second valve housing 176.

Valve housing 170 is preferably of two part construction. It includes afirst portion 178 and a second portion 180. Valve housing portions 178,180 are secured to cylinder head forming member 40, as shown in FIGS.1-3, 3-5, 7 and 15. In similar fashion, valve housing 176 is of two partconstruction. It includes a first part 182 and a second part 184. Bothare connected to cylinder head forming member 42, as shown in FIGS. 1,2-5, 10, 16, 17 and 17A.

Housings 170, 176 house valve components that together form adirection-of-conveying valve, i.e. a load/unload valve. This valve iscontrolled by a four-way valve 186 that is housed in housing part 180.It is a type of four-way control valve that includes a first port 188connected to the pressure line 156 and a second port 190 connected toreturn line 160. A suitable valve 186 is valve SV08-40 0-N-12D6,manufactured by Hydroforce Hydraulics Ltd. of Burmingham B6 5RX England.This valve is solenoid operated and so it can be remotely operated. Whenin a first position it connects a port 194 to pressure and a port 192 toreturn. When it is second position, it connects port 192 to return andport 194 to pressure.

As best shown by FIG. 32, a passageway 196 in housing member 178 extendsfrom port 194 to a pilot chamber 198 at the outer end of a valveassembly 200. As should be readily apparent, the valve spool assembly200 is constructed in a manner facilitating installation into andremoval from the valve housing portions 178, 180. Valve assembly 200includes an elongated valve spool cavity that has a portion 218 inhousing portion 178 and a portion 226 in housing portion 180.

Opposing sockets are formed in the housing parts 178, 180 where port 194joins passageway 196. A splice tube 202 has one end in one socket and asecond end in the other socket. Member 102 carries two surroundingO-rings, one for sealing between member 202 and the socket in part 180,and the other for sealing between member 202 and the socket in member178. In similar fashion, a pair of opposing sockets are formed in themembers 178, 180 where the four portions 218, 220, 224, 226 of the valvespool cavity meet. A splice tube 204 has a first end that fits snugly ina first of these sockets and a second end that fits snugly in the otherof the sockets. Member 204 like member 202 carries two O-rings. AnO-ring in O-ring groove 206 seals between member 204 and housing part180. An O-ring in O-ring groove 208 seals between member 204 and housingpart 178.

An opening 210 is formed in the left end (as pictured) of housing part178. It receives a closure plug 212. An O-ring is provided between theplug 212 and the opening 210, for sealing against fluid leakage at thislocation. The valve plug assembly includes a two position piston 214that reciprocates within a first region 216 of the valve plug cavity.Piston 214 includes a surrounding O-ring groove in which an O-ring isreceived, for sealing between piston 214 and the sidewall of cavity 216.An elongated cylindrical bore 218 extends from cavity 216 over to asecond cavity 220. Cavity 220 communicates with a cavity 222 in splicetube 204. Cavity 222 communicates with a passageway section 224 insplice tube 204. Passageway section 224 communicates with a passagewaysection 226 in housing part 180. Housing part 180 includes a cavity 228that communicates with passageway section 226 and a port 230 to which aport 232 is connected. Internally threaded socket opening is formed inan adjacent portion of housing part 180. Socket 233 receives anexternally threaded nipple 234 that is a part of a closure plug 236.Closure plug 236 includes an internal cavity 238 in which one end of aspring 240 is received.

The valve plug member is constructed in sections. It includes the piston214, a first rod portion 242, a poppet portion 246 having a valve seatcontacting surface on one end and a socket at its opposite end. An endportion of a second rod section 246 is received within the socket. Rodsection 246 includes a poppet head portion 248 that includes a valveseat contacting surface on one side an a socket portion on its otherside. This socket portion receives the second end of spring 240. Spring240 biases the valve spool member to the left, as illustrated in FIG.32. It pushes the valve seat contacting surface on valve plug section248 against a valve seat that is formed where passageway 226 meetscavity 228. The valve spool section 246 exerts an endwise force on valvespool section 242, pushing piston 214 against plug 212. Spring 240 alsobiases the valve seat contacting surface of valve plug 244 against avalve seat that is formed where passageway 218 intersects cavity 220.

Valve plug section 242 is inserted into the passageway 218 via thecavity 220. Then, the spliced members 202, 20 are inserted in thesockets formed for them in the housing parts 178, 180 and the housingparts 178, 180 are moved together. Then, piston 218 is inserted throughopening 210 and the plug 212 is screwed into place. At the opposite endof the assembly, valve spool section 246 is inserted into passageway 228via cavity 228. Then, the spring 240 and the closure plug 236 areinstalled. Also, the valve 186 is installed by screwing it into athreaded opening that is formed in housing part 180 to receive it. Thusassembled, the housing parts 178, 180 are attached to the cylinder headforming member 40, such as by use of bolts 250 (FIG. 1).

Valve housing 176 is constructed much like valve housing 170 except thatit does not include a control valve 186. Because of the commonconstruction, it will not be described in as much detail as valvehousing 170. The pilot chamber formed between the piston 214 and theplug 212 is connected to a port 252. A hose or pipe 254 extends fromport 192 in valve housing 170 to port 252 in valve housing 176. Whencontrol valve 182 is positioned such that the pressure port 188 isconnected with pressure port 192, there will be pressure in conduit 254and in port 252, and in the chamber between the plug 212 and the piston214. This pressure will move the valve plug assembly to the left,compressing the spring 240. This movement of the valve plug assembly242, 246 will communicate passageway 172 with passageways 256, 258, 262.

Housing parts 182, 184 include opposing sockets for receiving oppositeend pieces of a splice tube 264. Splice tube 264 includes O-ring groovesin its interior for receiving O-rings that seal between it and thehousing parts 182, 184. The assembled together housing parts 182, 184are connected to the head forming member 42, such as by the use of bolts266 (FIG. 1).

As best shown by FIGS. 17 and 17A, the cylinder head member 42 includesthree piston rod receiving openings 270, 272, 274 in which the pistonrods 24, 26, 28 are received. A manifold passageway 276 extends from aninlet/outlet port 278, into the member 42. A first segment of manifoldpassageway 276 extends from port 278 to one side of piston rod opening270. A second segment 276' extends from an opposite side of piston rodopening 270 to the first side of the next piston rod opening 272. Athird segment 276" extends from the second side of piston rod opening272 over to the adjacent side of piston rod opening 276. A port 280connects passageway segment 276" to port 262. Port 282 connects to port258 with manifold passageway segment 276'. Port 278 connects port 256with manifold passageway segment 276. Referring to FIGS. 21 and 32, port284 is connected to port 286. Port 286 is in communication with a firstportion of a manifold passageway 288. Port 290 communicates with asecond segment 288' of the manifold passageway. Port 232 communicateswith a third segment 288" of the manifold passageway.

As best shown in FIGS. 12, 14 and 18, 20, when the piston heads 104 aresubstantially against the cylinder head forming member 40, the girthgrooves in the end portions of the tubular part of piston rods 26 and 28are aligned with the manifold passageway segments in cylinder headforming member 40. In similar fashion, when the piston heads 106 aresubstantially against the cylinder head forming member 42, the girthgrooves in the end portions of member 102 for piston rods 26, 28 arealigned with the manifold passageway segments in the cylinder headforming member 42. When the piston heads 104, 106 are moved away fromthe cylinder head forming members 40, 42, full diameter portions of thepiston rod member 102 are in the cylinder rod receiving openings in thecylinder head forming members 40, 42, as shown by FIGS. 12 and 15-18.The girth groove adjacent piston head 104 is designed 292. The girthgroove adjacent piston head 106 is designated 294.

A sequence of operation will now be described with particular referenceto FIGS. 21-30. In FIGS. 21-30 the first variable volume workingchambers are designated a, b, c. The first inward variable volumeworking chambers are d, e, f. The second outward variable volume workingchambers are designated g, h, i. The second inward variable volumeworking chambers are-designated j, k, l.

The passageways that connect the working chambers a, b, c with theworking chambers g, h, i are designated p1, p2, p3 respectively. Thepassageways that connect the working chambers d, e, f with the workingchambers g, h, i are designated P4, P5, P6. The manifold passageways inthe cylinder head forming members 40, 42 are designated mp-1 and mp-2.The segments of manifold passageway mp-1 are designated mp-1a, mp-1b,mp-1c and mp-2a, mp-2b, mp-2c. The girth grooves are designated g1, g2and g3, g4.

FIG. 31 includes an arrow and the word "unload" for showing theunloading direction of a conveyor that includes the drive assembly 10.FIG. 21 shows the three piston rods 26, 28, 30 moving together, to movethe transverse drive beams 18, 20, 22 connected thereto, and theconveyor slats that are connected to the transverse drive beams 18, 20,22, in the "unload" direction. See FIG. 3 of the aforementioned U.S.Pat. No. Re. 35,022. In FIG. 21, the three passageways 256, 258, 263 areall connected to pressure "P" and the passageways 232, 290, 284 are allconnected to return "R". The switching valve SV (FIG. 31) is shifted todeliver pressure into conduit 172 and port 174. The pressure enterscavity 252 and moves the valve plugs 242, 246 to the left, compressingspring 240. The pressure then flows through the valve cavity and outthrough passageways 256, 258, 262 into the manifold segments mp-2a,mp-2b, mp-2c and then through the chamber ports into the workingchambers j, k, l. Some of the fluid pressure moves through thepassageways P1, P2, P3 up to the working chambers a, b, c. The fluidexpands the chambers j, k, l and a, b, c, moving both sets of pistons,and the piston rods 26, 28, 30, in the "unload" direction. Movementoccurs because simultaneously with the introduction of pressure into theworking chambers j, k, l, and a, b, c, the working chambers d, e, f, andg, h, i are connected to return "R." Fluid in working chambers d, e, f,moves out of these chambers through the chamber ports leading intomanifold segments mp-1a, mp-1b, mp-1c, and from there out through ports232, 290, 284 to the passageway in the valve and housing 170. At thattime, there is pressure in passageway 196 and in pilot chamber 198. Thispressure moves the piston 214 and the valve plugs to the right (asillustrated), moving the valve plugs away from the valve seats. Thisallows fluid entering through passageways 232, 290, 284 to flow throughthe valve and out passageway 168 and onto the return port 164 in theswitching valve SV. As the piston components move, fluid in workingchambers g, h, i, are forced out of these chambers and into passagewaysP4, P5, P6, an into first working chambers d, e, f, and then into themanifold passageway mp-1a, mp-1b, mp-1c and then out through thepassageways 232, 290, 284.

FIG. 22 shows the piston rods 26, 28, 30 fully extended. The girthgrooves g1, g2 are shown to be in alignment with the manifold passagewaysegments mp-1a, mp-1b, mp-1c. About simultaneous with the FIG. 22condition the switching valve SV is switched to connect passageway 166to pressure P and passageway 172 to return. FIG. 23 shows passageways232, 290, 284 connected to pressure P and shows passageway 256 connectedto return R. FIG. 31 shows that passageway 256 is connected topassageway 172 via cavity 254 in valve housing 176. The pressure frompassageways 232, 290, 284 enters manifold passageway segments mp-1a,mp-1b, mp-1c and is communicated via chamber ports with working chambersd, e, f, and via the passageways P4, P5, P6 with the working chambers g,h, i. However, at this time, the full diameter portion of piston rod 28that is within the piston rod receiving opening for piston rod 28 incylinder head forming member 42 blocks flow out from working chamber 1.Working chamber j is never blocked, and so piston rod 26 movesimmediately upon introduction of pressure into working chamber d andworking chamber g, via passageway P4. During movement of piston rod P1from its position shown in FIG. 22 to its position shown in FIG. 23, thefull diameter portion of piston rod 26 is within the piston rodreceiving opening for piston 26 and it blocks flow from manifoldpassageway segment mp-2b to manifold passageway segment mp-2a andpassageway 256. As a result, flow through passageways 258, 262 isblocked. When the piston rod 26 reaches the position shown in FIG. 23,the girth groove g3 connects manifold passageway segment mp-2b withmanifold passageway mp-2a. When this happens, there is a flow of fluidout through the chamber port for chamber k into the manifold passagewaymp-2b, and onto passageway 256 leading to return. FIG. 24 shows pistonrod 28 in the process of movement, owing to the flow of fluid out fromworking chamber k. It also shows the piston rod 28 substantiallyreaching full retraction, and girth groove g4 positioned to connectmanifold passageway segment mp-2c with manifold passageway segmentmp-2b. This allows fluid flow out from working chamber 1. In response tofluid movement out from working chamber 1, the piston rod 30 moves to aretracted position. FIG. 25 shows all three piston rods 26, 28, 30 fullyretracted. It is about this time that there is another switching ofswitching valve SV, to start the unloading cycle over again from thebeginning.

FIG. 26 shows all three piston rods 26, 28, 30, the three transversedrive beams 18, 20, 22 and all of the conveyor slats moving in unison inthe "load" direction. This requires operation of the four-way valve 186for the purpose of connecting passageway 254 with pressure andpassageway 196 with return. When passageway 196 is connected to return,the spring acting on the valve spools in housing 170 is free to move thevalve spools to the left. This is made possible by the piston 214 beingmoved to the left. Connection of passageway 254 with pressure connectspilot chamber 196 in valve housing 176 with pressure, causing a movementof piston 214 to the left, as illustrated. As it moves, piston 214 moveswith it the valve spools 242, 264, moving the two valve plugs out ofseated engagement of their valve seats. In response, pressure entersthrough passageway 254 and flows through the interior of the valve intothe passageways or ports 256, 258, 262, and onto the manifold passagewaysegments mp-2a, mp-2b, mp-2c. At the same time, pressure in thepassageways 232, 290, 280 acts on the valve plugs in housing 170, movingthem to the right, as illustrated. FIG. 27 shows the three piston rods26, 28, 30 fully advanced in the "load" direction. It further showspassageways 232, 290, 284 still connected to pressure and passageways256, 258, 262 still connected to return. About simultaneously with thepiston rods P1, P2, P3 reaching their fully advanced positions in the"load" direction, the switching valve SV is operated to again switchpressure and return between the two ports 162, 164. This connectspassageway 284 to return while passageways 256, 258, 262 are connectedto pressure. All three piston rods 26, 28, 30 want to retract. However,only piston rod 30 can retract because only chambers f, l are connectedto return. Chamber f is directly connected to return line 284 via thechamber port leading to manifold segment mp-1a. Chamber 1 is connectedto return via passageway 56 and then chamber f, the chamber port andmanifold passageway segment mp-1a. Piston rods 26, 28 cannot movebecause the fluid is trapped in chambers d, e, g, h. This is becausepassageways 290, 232 are blocked. However, once piston 30 is fullyretracted, its girth groove g2 connects manifold passageway segmentsmp-1a and mp-1b, allowing fluid to move out from chambers e, h, throughmanifold passageway segment mp-1b, girth groove 92, manifold passagewaysegment mp-1a and passageway 284. Fluid leaves chamber h throughpassageway 5 and chamber e.

When piston rod 28 is fully retracted, as shown in FIG. 29, girth grooveg1 connects manifold passageway segment mp-1b with manifold passagewaysegment mp-1c. This allows fluid to move from chamber j throughpassageway P4 into chamber d and move from chamber d through manifoldpassageway segment mp-1c, through girth groove g1 through manifoldpassageway segment mp-1b, through girth groove g2 through manifoldpassageway segment mp-1a, and into and through passageway 284, back toreturn. FIG. 30 shows the piston rod 26 substantially at the end of itsretract movement. Once it is fully retracted, all three piston rods 26,28, 30 again advance in the "load" direction, and the "load" sequence isrepeated.

There are some features of the invention that are usable with solidpiston rods. In particular, the sleeves 74, 92 can be used withadvantage with manifold blocks that are made primarily from aluminum andsolid piston shafts. Referring to FIG. 12, for example, the threecylinder tubes could be attached to one side of the manifold block inany suitable manner and the manifold block may be provided with socketson its opposite sides to receive three sleeves, one for each pistonshaft. As explained above, the sleeves are made from a stronger metalthan the metal used for the manifold blocks. The use of the sleevespermit the use of wear rings of substantial length, together with an oilseal and a dirt seal. The wear ring is in groove 78, closest to themanifold block. The oil seal is placed in the center groove 76. The dirtseal is placed in the end groove 82. The sleeve and wear ring guide theshaft in a substantially true path where it moves through the manifoldblock. Thus, the shaft does not impose any great wear on the manifoldblock.

The illustrated embodiments are only examples of the present inventionand, therefore, are non-limitive. It is to be understood that manychanges in the particular structure, materials and features of theinvention may be made without departing from the spirit and scope of theinvention. Therefore, it is my intention that my patent rights not belimited by the particular embodiments illustrated and described herein,but rather determined by the following claims, interpreted according toaccepted doctrines of claim interpretation, including use of thedoctrine of equivalents and reversal of parts.

What is claimed is:
 1. A tandem linear hydraulic drive unit,comprising:first and second longitudinally spaced apart cylinder heads;first and second tubular sidewalls, each having an inward end and anoutward end; said inward end of said first tubular sidewall beingconnected to the first cylinder head; said inward end of said secondtubular sidewall being connected to said second cylinder head; a firstoutward end wall at the outward end of the first tubular sidewall; asecond outward end wall at the outward end of said second tubularsidewall; a first space formed in and by the first tubular sidewall,axially between the first cylinder head and the first outward end wall;a second space formed in and by the second tubular sidewall, axiallybetween the second cylinder head and the second outward end wall; afirst piston head within the first space, axially dividing it into afirst outward variable volume working chamber and a first inwardvariable volume working chamber; a second piston head in the secondspace, axially dividing it into a second outward variable volume workingchamber and a second inward variable volume working chamber; anelongated piston rod having a first end and a second end, said first endbeing connected to the first piston head within the first space and thesecond end being connected to the second piston head within the secondspace; said first cylinder head including an opening through which afirst end portion of the piston rod extends; said second cylinder headincluding an opening through which a second end portion of the pistonrod extends; a first passageway extending from the first outwardvariable volume working chamber, through the first piston head, thepiston rod and the second piston head to the second inward variablevolume working chamber; a second passageway extending from the firstinward variable volume working chamber through the first piston head,the piston rod and the second piston head to the second outward variablevolume working chamber; said first cylinder head including aninlet/outlet port, a first manifold passageway extending inwardly of thecylinder head from the inlet/outlet port, and a chamber port connectingthe first manifold passageway to the first inward variable volumeworking chamber; and said second cylinder head including an inlet/outletportion, a second manifold passageway extending inwardly of the cylinderhead from the inlet/outlet portion, and a chamber port connecting thesecond manifold passageway to the second inward variable volume workingchamber.
 2. A tandem drive unit according to claim 1, wherein the firstcylinder head includes an internally threaded first socket positioned toreceive the inward end of the first tubular sidewall, and an internallythreaded second socket on the side of the first cylinder head oppositethe internally threaded first socket, wherein the inward end of thefirst tubular sidewall has external threads that mate with the internalthreads in the internally threaded first socket, and said tandemcylinder includes a tubular sleeve having an externally threaded firstend that fits into and mates with the internally threaded second socket,said sleeve carrying a seal that surrounds the first end portion of thepiston rod and seals between the sleeve and the piston rod.
 3. A tandemdrive unit according to claim 1, wherein the second cylinder headincludes an internally threaded first socket positioned to receive theinward end of the second tubular sidewall, and an internally threadedsecond socket on the side of the second cylinder head opposite theinternally threaded first socket, wherein the inward end of the secondtubular sidewall has external threads that mate with the internalthreads in the internally threaded first socket, and said tandemcylinder includes a tubular sleeve having an externally threaded firstend that fits into and mates with the internally threaded second socket,said sleeve carrying a seal that surrounds the second end portion of thepiston rod and seals between the sleeve and the piston rod.
 4. A tandemdrive unit according to claim 1, where the outward end of the firsttubular sidewall is threaded and the first outward end wall has athreaded portion that mates with the threads on the outward end of thefirst tubular sidewall, for connecting the first outward end wall to theoutward end of the first tubular sidewall.
 5. A tandem drive unitaccording to claim 1, where the outward end of the second tubularsidewall is threaded and the second outward end wall has a threadedportion that mates with the threads on the outward end of the secondtubular sidewall, for connecting the second outward end wall to theoutward end of the second tubular sidewall.
 6. A tandem drive unitaccording to claim 1, wherein the outward ends of the tubular sidewallsare internally threaded and the outward end walls have externallythreaded portions that thread into the threads in the outward ends ofthe tubular sidewalls.
 7. A tandem drive unit according to claim 1,wherein the elongated piston rod is tubular and is formed by an annularsidewall, and wherein a smaller diameter tube that also has a sidewallextends axially through the piston rod, wherein the interior of thesmaller diameter tube forms one of the first and second passageways, atleast in its extend through the piston rod, and an annular space betweenthe sidewall of the piston wall and the sidewall of the inner tube formsthe other passageway, at least in its extend through the piston rod. 8.A tandem drive unit according to claim 1, wherein at least one of thecylinder heads includes an elongated opening in which a portion of thepiston rod is received, and includes a fluid passageway intersectingsaid opening on one side of said opening and further includes anotherfluid passageway intersecting the opening at a location spaced aroundthe opening from the first fluid passageway, wherein a portion of thepiston rod that moves through said opening includes a recess thatextends at least partially around the piston rod, wherein the piston rodis positionable to place the recess into registry with the two fluidpassageways, so that fluid in one passageway can travel from it throughthe recess to and through the other passageway, and wherein the pistonrod is also positionable to place another portion of the piston rod thatmoves through the opening, and which is cylindrical, and blockscommunication from one of the passageways to the other.
 9. Incombination, three side-by-side tandem linear hydraulic drive units,each comprising:first and second longitudinally spaced apart cylinderheads; first and second tubular sidewalls, each having an inward end andan outward end; said inward end of said first tubular sidewall beingconnected to the first cylinder head; said inward end of said secondtubular sidewall being connected to said second cylinder head; a firstoutward end wall at the outward end of the first tubular sidewall; asecond outward end wall at the outward end of said second tubularsidewall; a first space formed in and by the first tubular sidewall,axially between the first cylinder head and the first outward end wall;a second space formed in and by the second tubular sidewall, axiallybetween the second cylinder head and the second outward end wall; afirst piston head within the first space, axially dividing it into afirst outward variable volume working chamber and a first inwardvariable volume working chamber; a second piston head in the secondspace, axially dividing it into a second outward variable volume workingchamber and a second inward variable volume working chamber; anelongated piston rod having a first end and a second end, said first endbeing connected to the first piston head within the first space and thesecond end being connected to the second piston head within the secondspace; said first cylinder head including an opening through which afirst end portion of the piston rod extends; said second cylinder headincluding an opening through which a second end portion of the pistonrod extends; a first passageway extending from the first outwardvariable volume working chamber, through the first piston head, thepiston rod and the second piston head to the second inward variablevolume working chamber; a second passageway extending from the firstinward variable volume working chamber through the first piston head,the piston rod and the second piston head to the second outward variablevolume working chamber; wherein the first cylinder heads for all threetandem cylinders are combined in a first cylinder head forming member;wherein the second cylinder heads for all three tandem cylinders arecombined in a second cylinder head forming member; said first cylinderhead forming member comprising a first manifold passageway having aninlet/outlet port, and a separate chamber port connecting the manifoldpassageway with each first inward variable volume working chamber; andsaid second cylinder head forming member comprising a second manifoldpassageway having an inlet/outlet port, and a separate chamber portconnecting the manifold passageway with each second inward variablevolume working chamber.
 10. The combination of claim 9, wherein thesecond cylinder head forming member includes three parallel, axiallyelongated openings in which adjacent end portions of the three pistonrods are received, and wherein the inlet/outlet port for the secondmanifold passageway is at a first end of the second cylinder headforming member adjacent a first tandem cylinder, and said secondmanifold passageway extends inwardly from its inlet/outlet port andcrosswise of the openings in which the piston rods for the first andsecond tandem cylinders are located, each said piston rod having arecessed portion that is aligned with the second manifold passagewaywhen the said piston rods are positioned to place the second pistonheads on said piston rods in an end position closely adjacent the secondcylinder head forming member, and wherein the piston rods are movableaway from said position of alignment of the recesses with the secondmanifold passageway, into positions in which the piston rods block thesecond manifold passageway and prevent fluid flow through the secondmanifold passageway across the openings in which said piston rods arereceived.
 11. The combination of claim 10, wherein the first cylinderhead includes an internally threaded first socket positioned to receivethe inward end of the first tubular sidewall, and an internally threadedsecond socket on the side of the first cylinder head opposite theinternally threaded first socket, wherein the inward end of the firsttubular sidewall has external threads that mate with the internalthreads in the internally threaded first socket, and said tandemcylinder includes a tubular sleeve having an externally threaded firstend that fits into and mates with the internally threaded second socket,said sleeve carrying a seal that surrounds the first end portion of thepiston rod and seals between the sleeve and the piston rod.
 12. Thecombination of claim 11, wherein the second cylinder head includes aninternally threaded first socket positioned to receive the inward end ofthe second tubular sidewall, and an internally threaded second socket onthe side of the second cylinder head opposite the internally threadedfirst socket, wherein the inward end of the second tubular sidewall hasexternal threads that mate with the internal threads in the internallythreaded first socket, and said tandem cylinder includes a tubularsleeve having an externally threaded first end that fits into and mateswith the internally threaded second socket, said sleeve carrying a sealthat surrounds the second end portion of the piston rod and sealsbetween the sleeve and the piston rod.